Added the first means to pass information between 2 tasks using the

utcb as a shared page instead of the message registers.

Implemented the code that passes task information from mm0 to fs0
using the fs0 utcb. The code seems to work OK but:

There's an issue with anon pages that they end up on the same swapfile
and with same file offsets (e.g. utcb and stack at offset 0). Need to
fix this issue but otherwise this implementation seems to work.

TODO:
- Separate anon regions into separate vmfiles.
- Possibly map the stacks from virtual files so that they can be
  read from userspace in the future for debugging.
- Possibly utcb could be created as a shared memory object using shmget/shmat
  during startup.

tasks/fs0/src/task.c

@@ -35,7 +35,7 @@ struct tcb *find_task(int tid)
  * registers are sufficient. First argument tells how many there are, the rest
  * tells the tids.
  */
-int receive_pager_taskdata(l4id_t *tdata)
+int receive_pager_taskdata_orig(l4id_t *tdata)
 {
 	int err;
 
@@ -69,15 +69,51 @@ int receive_pager_taskdata(l4id_t *tdata)
 	return 0;
 }
 
+struct task_data {
+	unsigned long tid;
+	unsigned long utcb_address;
+};
+
+struct task_data_head {
+	unsigned long total;
+	struct task_data tdata[];
+};
+
+/* Read task information into the utcb buffer, since it wont fit into mrs. */
+struct task_data_head *receive_pager_taskdata(void)
+{
+	int err;
+
+	/* Ask pager to write the data at this address */
+	write_mr(L4SYS_ARG0, (unsigned long)utcb->buf);
+
+	/* Make the actual ipc call */
+	if ((err = l4_sendrecv(PAGER_TID, PAGER_TID,
+			       L4_IPC_TAG_TASKDATA)) < 0) {
+		printf("%s: L4 IPC Error: %d.\n", __FUNCTION__, err);
+		return PTR_ERR(err);
+	}
+
+	/* Check if call itself was successful */
+	if ((err = l4_get_retval()) < 0) {
+		printf("%s: Error: %d.\n", __FUNCTION__, err);
+		return PTR_ERR(err);
+	}
+
+	printf("%s: %d Total tasks.\n", __FUNCTION__,
+	       ((struct task_data_head *)utcb->buf)->total);
+
+	return (struct task_data_head *)utcb->buf;
+}
+
 /* Allocate a task struct and initialise it */
-struct tcb *create_tcb(l4id_t tid)
+struct tcb *create_tcb(void)
 {
 	struct tcb *t;
 
 	if (!(t = kmalloc(sizeof(*t))))
 		return PTR_ERR(-ENOMEM);
 
-	t->tid = tid;
 	INIT_LIST_HEAD(&t->list);
 	list_add_tail(&t->list, &tcb_head.list);
 	tcb_head.total++;
@@ -85,31 +121,37 @@ struct tcb *create_tcb(l4id_t tid)
 	return t;
 }
 
-int init_task_structs(l4id_t *tdata)
+int init_task_structs(struct task_data_head *tdata_head)
 {
 	struct tcb *t;
-	int total = tdata[0];
 
-	for (int i = 0; i < total; i++) {
-		if (IS_ERR(t = create_tcb(tdata[1 + i])))
+	for (int i = 0; i < tdata_head->total; i++) {
+		if (IS_ERR(t = create_tcb()))
 			return (int)t;
+
+		/* Initialise fields sent by pager */
+		t->tid = tdata_head->tdata[i].tid;
+		t->utcb_address = tdata_head->tdata[i].utcb_address;
+
+		/* Initialise vfs specific fields. */
 		t->rootdir = vfs_root.pivot;
 		t->curdir = vfs_root.pivot;
 	}
+
 	return 0;
 }
 
 int init_task_data(void)
 {
-	l4id_t tdata[MR_UNUSED_TOTAL];
+	struct task_data_head *tdata_head;
 
 	INIT_LIST_HEAD(&tcb_head.list);
 
 	/* Read how many tasks and tids of each */
-	BUG_ON(receive_pager_taskdata(tdata) < 0);
+	BUG_ON((tdata_head = receive_pager_taskdata()) < 0);
 
 	/* Initialise local task structs using this info */
-	BUG_ON(init_task_structs(tdata) < 0);
+	BUG_ON(init_task_structs(tdata_head) < 0);
 
 	return 0;
 }